This invention concerns a continuous casting method with a pulsating magnetic field along the crystalliser and the relative crystalliser for continuous casting.
The invention is applied to machines performing continuous casting of billets, blooms and slabs, particularly thin slabs, in the field of the production of iron and steel.
The state of the art of the continuous casting field covers the use of electromagnetic devices associated externally with the sidewalls of a crystalliser and able to generate an electromagnetic field interacting with the molten metal being cast.
In the state of the art this electromagnetic field mainly has the purpose of improving the surface quality of the product, principally by acting on the liquid metal so as to improve the characteristics of solidification; another purpose is to displace the surface of the molten metal in the zone of the joint between the refractory material and the crystalliser so that the solidification begins only in the crystalliser and there are no leakages of material.
The electromagnetic devices of the state of the art normally comprise a coil or one single inductor positioned in cooperation with the outside of the wall of the crystalliser and generally close to the zone of the beginning of solidification of the metal.
Embodiments have been disclosed in which the coil or inductor generates a stationary alternating magnetic field (see the article "Improvement of Surface Quality of Steel by Electromagnetic Mold" taken from the documents of the International Symposium on the "Electromagnetic Processing of Materials"--Nagoya 1994) or else generates an alternating magnetic field modulated in amplitude (see the article "Study of Meniscus Behavior and Surface Properties During Casting in a High-Frequencies Magnetic Field" taken from "Metallurgical and Materials Transaction"--Vol. 26B, April 1995).
Other embodiments disclosed provide for the magnetic field generated to be periodically pulsating with waves defined by successions of pulses of a substantially constant amplitude (U.S. Pat. No. 4,522,249) or else for the magnetic field to be generated by electromagnetic waves of a development which is attenuated until it is eliminated within a half-period (SU-A-1021070 and SU-A-1185731).
Experimental tests have shown that such configurations of the electromagnetic field acting in the crystalliser are not suitable to achieve the desired results in view of the different conditions which occur within the solidifying metal.
These different conditions, which are due to the different physical state and different temperature of the solidifying metal, cause an interaction between the magnetic field and the metal, this interaction being different from one zone of the crystalliser to the other and therefore not the best along the whole length of the crystalliser.
Moreover, in the state of the art, there are problems in the connection between the inductors outside the crystalliser and the crystalliser itself as regards dispersions in and attenuations of the electromagnetic field generated, which causes a reduction in the intensity of the forces acting on the molten metal.
There is also the problem of the mechanical deformation to which the inductors may be subjected during use.
Particularly, but not only, the state of the art does not make possible to fulfill the following functions:
to reduce the friction between the cast product and the crystalliser by inducing pulsating forces directly onto the solid skin, and also onto the liquid part where that is necessary, in order to increase the casting speed;
not to use the traditional systems of mechanical oscillation of the ingot mold with a consequent improvement of the surface quality of the product, as the oscillation marks are eliminated;
to control the effect on the meniscus according to the requirements of the process so as to improve both the lubrification of the sidewall and also the surface quality and the inner quality of the product;
to use the capacity of resonance of the solidified skin and the skin-liquid system so as to improve the heat exchnage in the mushy zone in order to encourage a growth of the product with an equal axis and a consequent improvement in the inner quality;
to use the migrating field configuration so as to induce in the liquid part a vertical stirring (direction of the axis of the crystalliser) so as to obtain an optimum effect;
to improve the heat exchange in the lower part of the crystalliser where the skin is separated from the crystalliser, thus increasing the total quantity of the heat removed by the crystalliser, thus making it possible to achieve higher casting speeds and improvements in the quality of the product.